We have a large datawarehouse database on Oracle and are encountering
strange performance problems. A query such as "select count(*) from
dba_data_files;" will run in a few seconds or as long as 15 minutes
depending on when this is run. We've narrowed it down to where it is
not even directly related to load on the DB. A few days ago, I ran this
query right after we started the DB (no activity in the DB at all) and
it took 6 minutes! The waits we were seeing were "db file sequential
read" and "buffer busy wait" for this query. Keep in mind our DB is
~27TB with ~30,000 datafiles!